Orientation And Characteristics Of Fractures In Crystalline Bedrock Determined By Surface And Borehole Geophysical Surveys, Millville And Uxbridge, Massachusetts

Abstract

Four geophysical techniques were used to determine bedrock fracture
orientation and other site characteristics that can be used to determine
ground-water flow and contaminant transport at a study area underlain by
fractured crystalline bedrock in Millville and Uxbridge, Massachusetts. In the
study area, azimuthal seismic-refraction and azimuthal square-array
direct-current resistivity surveys were conducted at three sites, borehole-radar
surveys were conducted in a cluster of three wells, and ground-penetrating radar
surveys were conducted along roads.

Azimuthal seismic-refraction data indicated a primary fracture strike ranging
from 56 to 101 degrees at the three sites. Graphical and analytical analysis of
azimuthal square-array resistivity data indicated a primary fracture strike
ranging from 45 to 90 degrees at the same three sites, Directional
borehole-radar data from three wells indicated 46 fractures or fracture zones
located as far as 147 feet from the surveyed wells. Patterns of low radar-wave
velocity and high radar-wave attenuation from cross-hole radar surveys of two
well pairs were interpreted as a planai fracture zone that strikes 297 degrees
and dips 55 degrees south. Ground-penetrating radar surveys with IOO-MHz
antennas penetrated 5 to 50 feet of unconsolidated overburden and as much as 60
feet of bedrock where the bedrock surface was at or near land surface.
Horizontal and subhorizontal fractures were observed on the ground-penetrating
radar records at numerous locations. Comparison of results from the various
geophysical techniques indicates good agreement and indicates primary high-angle
fracturing striking east-northeast. Square-array resistivity data yielded an
average secondary bedrock porosity of 0.0044 and an average aperture of 0.007 1
foot for high-angle fractures.

Final copy as submitted to Symposium on the Application of Geophysics to Engineering and
Environmental Problems for publication as: Hansen, B.P., and Lane, J.W., Jr., 1996, Orientation and characteristics of
fractures in crystalline bedrock determined by surface and borehole geophysical
surveys, Millville and Uxbridge, Massachusetts, in Bell, R.S., and Cramer, M.H.,
eds., Symposium on the Application of Geophysics to Engineering and
Environmental Problems, Keystone, Colo., April 28-May 2, 1996, Proceedings:
Wheat Ridge, Colorado, Environmental and Engineering Geophysical Society, p.
927-940.